This invention relates to a sewing machine in general and more particularly to a lower thread supplying mechanism in a sewing machine.
With the conventional sewing machines, a stitch is formed on a fabric by interlocking an upper thread carried by a vertically reciprocating needle and a lower thread wound around a bobbin contained in a loop-taker. The loop-taker is mounted below a needle plate having a needle penetrating hole and operated in synchronism with reciprocation of the needle to interlock the upper thread with the lower thread in a well known manner.
There has been proposed a lower thread drawing means arranged substantially on the travelling path of the lower thread between the bobbin and the needle hole. The said means is operated in synchronism with the loop-taker and includes a bar-like member adapted to move up and down whereby a predetermined amount of the lower thread is drawn out of the bobbin during the downward movement of the bar-like member, against a biasing force of a lower thread tension spring attached to the bobbin. The lower thread drawing operation is effected while the fabric is not being fed. The lower thread which has thus been pulled by the bar-like member will be supplied to the fabric as the fabric is being fed for the subsequent stitching operation.
Disadvantageously, such prior art arrangement will have a tendency to pull back the lower thread which has been interlocked with the upper thread to form a stitch on the fabric, resulting in shrinkage or crumpling of the stitch, as well as failure to supply a predetermined amount of the lower thread. This tendency will be increased as the lower thread tension spring is adjusted to have a greater biasing force to the lower thread which is just draw out of the bobbin.